1. Field of the Invention
The present invention relates an allocation of resources, i.e., frequency and power, to terrestrial services and satellite services that use a same frequency band.
2. Description of the Prior Art
A satellite radio service is a communication service in which a user employs a device that is in direct radio frequency (RF) communication with a satellite. Typically, the satellite is also in communication with a terrestrial system known as an earth station or gateway, which is, in turn, coupled to a public service telephone network (PSTN). Satellite TV service and mobile satellite service are examples of satellite radio services.
A terrestrial radio service is a communication service in which a user employs a device that is in direct RF communication with a terrestrial base station (TBS). Conventional cellular telephone service is an example of a terrestrial radio service.
Satellite radio services and terrestrial radio services currently use frequencies in two ways:
(i) They use separate frequency bands. This means that the satellite radio services use a specific frequency band and the terrestrial radio service uses a different frequency band.
(ii) When the terrestrial services use directive antennas, i.e., antennas that point the transmitted energy in a particular direction, the terrestrial service and the satellite service can use the same frequency. This can be done because the directive antennas used terrestrially put the energy at low elevation angles, i.e., along the horizon, to another directive antenna. Accordingly, the amount of energy transmitted towards a satellite in space is small, and does not ordinarily interfere with the satellite service. The satellite service has imposed on it a power flux density limit and the limit usually varies with elevation angle. At low elevation angles, the amount of power that a satellite can radiate is small. At high elevation angles, the amount of energy is larger. In this way, a satellite transmission does not interfere with the terrestrial transmission.
When the terrestrial service uses an omni-directional antenna, e.g., an antenna that transmits energy over a hemisphere, terrestrial and satellite services cannot share the same frequency bands because the two services would be prone to interfere with one another. For example, satellites do not use the same frequencies as are used with cellular services because a cellular handset has an omni type antenna.
There are four interference scenarios when satellite and terrestrial use the same frequency, particularly when the terrestrial service is mobile.
The first interference scenario is where a ground based satellite transmitter, i.e., a handset transmitting to a satellite, interferes with a receiving TBS, e.g., a cell tower. The second interference scenario is where a transmitting TBS interferes with a ground-based satellite receiver, e.g., a handset receiving from a satellite. One technique for avoiding these two types of interference is for the ground based satellite unit, e.g., the handset, to not use the same frequency as the TBS when the ground-based satellite unit is in an area near the TBS. For cellular and like services, this area is typically regarded as being within a radius of 1 to 7 kilometers from the TBS.
The third interference scenario is a satellite transmitter, in space, interfering with a terrestrial unit, e.g., a handset receiving a transmission from a TBS. A technique for avoiding this form of interference is to use a power flux density limit on the satellite.
The fourth interference scenario is a transmission from a terrestrial unit, e.g., a handset transmitting to a TBS, interfering with a satellite receiver in space. The satellite receiver could receive each terrestrial unit's transmission, especially if the terrestrial unit is using an omni-type antenna.
The satellite beam covers an area of the Earth's surface, typically 1000 to 1,000,000 square miles, but the area could be larger. In comparison a TBS beam covers a relatively smaller area of 1 to 60 square miles. However, if the TBS is located in the area covered by the beam of the satellite, then when the TBS uses a particular frequency, the satellite is denied from using that frequency in the full area covered by the satellite beam. So, for example, although the TBS covers an area of only 1 to 60 square miles, the satellite service is denied use of the frequency in an area of 1000 to 1,000,000 square miles.